As the Syrian Civil War has waged on over the past few years, the United States, along with Arab states such as Saudi Arabia and Qatar, decided to arm the Syrian rebels under the “Free Syrian Army” umbrella.

The shipments of former Yugoslavian Army weapons held by the Croatians, along with newly manufactured Croatian weapons, began in November 2012, according to The Telegraph UK. The weapons were flown out of Croatia on chartered flights to Jordan and then smuggled into Syria.

Fast forward to the rise of ISIS and their blitzkreig campaign across northern Iraq. An investigation by the arms control organization Conflict Armament Research has found that various U.S. supplied weapons have wound up in the hands of ISIS.

The study by the London-based small-arms research organisation Conflict Armament Research documented weapons seized by Kurdish forces from militants in Iraq and Syria over a 10-day period in July.

The report said the jihadists disposed of “significant quantities” of US-made small arms including M16 assault rifles and included photos showing the markings “Property of US Govt”.

It also found that anti-tank rockets used by IS in Syria were “identical to M79 rockets transferred by Saudi Arabia to forces operating under the Free Syrian Army umbrella in 2013”.

The U.S.-made small arm in question is the M16A4 assault rifle. It is the main service rifle of the U.S. Marine Corps and sees limited use in the U.S. Army. More importantly, thousands of them were supplied to the Iraqi Army. The source of ISIS's M16s, along with other U.S. made weapons, is likely captured Iraqi Army stocks.

As for the M79s, they were part of the arms shipments from Croatia that were meant for the Free Syrian Army. The weapons were either captured, or acquired by former FSA fighters as they joined ISIS.

The lesson to be learned here is that foreign policy decisions often have unintended consequences. As the U.S. contemplates what future action it will take to combat ISIS, it needs to be sure to learn those lessons of the past.